The present invention relates to calibration of oscilloscope deflection and particularly to automatic calibration accomplished without utilizing a specialized cathode-ray-tube or requiring manual adjustment.
Since an oscilloscope is usually employed as an accurate measuring instrument, the accurate calibration thereof is essential. One calibration consideration relates to the accuracy of the deflection of the electron beam in the oscilloscope's cathode-ray-tube so that the amplitude (Y deflection) and duration (X deflection) of a tested waveform are accurately portrayed. Heretofore, it has been the practice to employ a calibrated standard as a deflection waveform input for the oscilloscope, and then adjust the deflection factor or amplification of the oscilloscope circuitry so that a predetermined deflection is achieved. For instance, a common requirement in oscilloscope specifications is that a 50 millivolt increment in input signal be required to move the cathode-ray-tube's electron beam between graticule divisions on the tube's faceplate. Not only is down time consumed for the periodic calibration of the oscilloscope, but also some skill on the part of maintenance personnel is necessary for proper adjustment.
It is possible to adjust the positioning of a cathode-ray-tube's electron beam automatically so that its location can be predetermined. For example, beam indexing cathode-ray-tubes employed for color television applications can accurately locate the tube's electron beam so as to register properly with the separate color emitting phosphors of the primary colors. U.S. Pat. Nos. 4,247,869; 2,778,971 and 2,790,107 and British patent No. 822,017 illustrate cathode-ray-tubes employing indexing elements, strips or bands supplying feedback indicative of the electron beam's actual position. The indexing elements produce a light emission or electron emission when crossed by the electron beam and this emission is detected within the tube in order to position the beam. Unfortunately, the manufacture of such tubes requires sophisticated and complex screening techniques.